1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus comprising a plurality of developing devices and a plurality of photoreceptors.
2. Description of Related Art
A color image forming apparatus forms a color image by forming a yellow (Y) toner image, a magenta (M) toner image, a cyan (C) toner image and a black (K) toner image on a plurality of photosensitive drums respectively and by combining the Y, M, C and K toner images on an intermediate transfer belt.
In such an image forming apparatus, in order to prevent non-coincidence of Y, M C and K toner images in a main-scanning direction, image stabilization control is performed. In the image stabilization control, test patterns, each composed of a horizontal line (a line extending in the main-scanning direction) and a diagonal line, are formed, and positional errors in the main-scanning direction are detected based on the gaps between the horizontal lines and the diagonal lines of the respective test patterns. Then, in accordance with the detected positional errors, the image forming apparatus makes corrections to the image forming positions of Y, M, C and K toner images.
However, this image stabilization control is not enough to prevent non-coincidence of colors. FIG. 6 shows the test patterns formed on the intermediate transfer belt in the image stabilization control.
Generally, photosensitive drums actually do not rotate at a constant speed because the gears for rotation of the photosensitive drums have production errors. Accordingly, as shown by FIG. 6, even if Y, M C and K test patterns, each composed of a horizontal line and a diagonal line, coincide with one another in the main-scanning direction, the gaps ΔY, ΔM, ΔC and ΔK between the horizontal lines and the diagonal lines in the Y, M, C and K test patterns depend on in what parts of the respective photosensitive drums with respect to the sub-scanning direction the Y, M, C and K test patterns were formed. This prevents accurate detection of positional errors of the Y, M, C and K test patterns in the main-scanning direction. Thus, with the image stabilization control described above, it is impossible to fully prevent color registration errors.
In order to improve the detection accuracy of positional errors of test patterns despite the eccentricity and the inconstant-speed rotation of the photosensitive drums, for example, Japanese Patent Laid-Open Publication No. 11-65208 suggests a color image forming apparatus that forms a plurality of toner marks and that calculates a median error. Thus, some inventions have been made to minimize the effects of the inconstant-speed rotation of the photosensitive drums on the detection of positional errors in the main-scanning direction of test patterns.